This invention relates to the field of abrasive or sanding disks, and in particular this invention relates to backing plates for abrasive disks and accessories for angle grinders and means for making them.
Abrasive disks, or sanding disks are widely used on portable electric drills and (at a more professional level) on hand-held angle grinders. When used on these machines the disk is held by its centre against a backing plate and is rotated at generally a high speed while pressed in front of a backing plate against the work. The abrasive surface wears down the surface of the work by, in effect, a cutting action. Angle-grinder mounted sanding disks are commonly used (for example) in automotive panel beating, where body filler is to be sanded back to conform to the original contours of a remodelled car part. It is said that millions of sanding disks suitable for use with angle grinders are sold each year. There are some problems related to the use of sanding disks, such as:
(a) The relatively rigid backing plates commonly used with angle grinder sanding disks force the sanding disks into an unsatisfactory mode of operation when the angle grinder is tilted towards the work during usexe2x80x94such as that primarily the edge engages with the work, resulting in local, intense action rather than an even, gradual action over a wider area. There is a tendency for the work surface to develop an unsatisfactory scalloped surface which requires hand sanding block treatment. The disks cannot be used for finely controlled work such as preparation of surfaces in a state ready for painting.
(b) Sometimes the material being abraded tends to melt at the high cutting speeds involved, and if this happens it is particularly likely to clog the sanding disk in a quick and effective manner so that the disk has to be discarded. Melting may also lead to the tool biting in and as a result the surface of the work may be inadvertently destroyed. Heating also adversely affects the life of the sanding disk.
(c) The operator cannot see the material being sanded during the actual operation; he/she can only see material that is not covered by the blade. It is difficult to carry out a precise operation without repeatedly inspecting the work in progress and more closely reaching an approximation to the desired result. Hand-held tools cannot be re-applied precisely so that repeated inspection is not a good option for careful work.
It is a well known phenomenon that a disk having perforations becomes semi-transparent when spun at a moderate to high speed because of the persistence of image on the retina in the human eyexe2x80x94the xe2x80x9cpersistence of visionxe2x80x9d effect. The image seen through a perforated spinning disk is further enhanced if there is a contrast in light and/or colour between the spinning disk and its background and/or foreground. To increase the width of the xe2x80x9cwindowxe2x80x9d or see-through viewing effect when a disk is spun, perforations are usually designed to overlay each other. There are many abrasive and rasping disks that make use of this phenomenon. Examples are those of F. Reidenback filed Aug. 31, 1953 U.S. Pat. No. 2,749,681 or J. C. Schwartz filed Mar. 26, 1985 U.S. Pat. No. 4,685,181.
Because of the presumed catastrophic consequences of protrusions into large apertures of perforated disks these inventions to date have relied on using many small perforations in the disk in relation to total disk size.
Although we relate the invention to angle grinders in particular, the invention is also applicable to sanding disks used in some other power tools, such as ordinary electric drills, even though the usual types of electric drills do not spin at such a high speed.
xe2x80x9cAperturexe2x80x9d means a channel or hole passing completely through an object, and is surrounded on all sides by the material of the object. It is not limited to apertures having a circular profile.
xe2x80x9cDishedxe2x80x9d means that a disk has been formed into a convex shape (like a saucer) and for this invention the abrasive would usually be found on the base, or convex side, of the saucer.
xe2x80x9cDiskxe2x80x9d refers to a flat piece of relatively rigid material (though having some resilience) which is adapted for mounting on a rotatable spindle or arbor. It is not limited here to purely circular shapes and the materials used can be any of those known for use in the production of abrasive disks for rotary grinders.
xe2x80x9cGapxe2x80x9d means an indentation or invagination which is incompletely surrounded by the material of the object. It would include therefore configurations in which the circular periphery of a disk has had a segment, (defined below), removed or the configuration obtained by (notionally) moving an xe2x80x9caperturexe2x80x9d until a portion extended beyond the periphery of the disk.
xe2x80x9cSandingxe2x80x9d is used herein to refer to any abrading or finishing operation in which the surface of a workpiece is treated to remove material or alter the roughness.
xe2x80x9cSegmentxe2x80x9d means that portion of a circle which lies between the perimeter and a chord.
In a first broad aspect the invention comprises a backing plate for use as part of a sanding system for use with an angle grinder or the like, comprising a backing plate and a disk bearing at least one abrasive surface, the disk being adapted for mounting upon an arbor of the angle grinder in conjunction with a matching backing plate, characterized in that the sanding disk is modified by being provided with at least one non-concentric aperture adapted for viewing and ventilation which aperture is capable in use of being substantially in alignment with at least one similarly adapted viewing and ventilation gap or aperture constructed within the backing plate, so that in use the work surface and the sanding disk are cooler as a result of air movement, abraded material is moved tangentially away, and the user can see the work through the at least one non-concentric apertures.
The term xe2x80x9cnon-concentricxe2x80x9d as applied to apertures in this Application means that the aperture is displaced from the axis of rotation along a radius of the disk. A preferred number of non-concentric apertures adapted for viewing and ventilation is between one and nine. A more preferred number of non-concentric apertures is between three and five.
Preferably the non-concentric apertures adapted for viewing and ventilation are placed at varying distances from the centre of rotation of the disks, so that when the disk is rotated, a substantial proportion of the area beneath the disk can be seen.
The Abrasive Disk Component
The abrasive disk component of the sanding system is the subject of PCT/US96/19191 but is described herein to assist in understanding the invention claimed herein which relates to the backing plate used in conjunction with such abrasive disks.
The sanding disk as described previously, can be modified to provide that at least one edge of the or each non-concentric aperture adapted for viewing and ventilation is formed in order to serve as a cutting edge.
In a further aspect the viewing or ventilation apertures may also be regarded as means to intermittently interrupt the abrading action of the disk as it turns, thereby providing a xe2x80x9crest timexe2x80x9d during which time the work surface may become cooler.
In another aspect the sanding disk may be provided with one or more apertures primarily intended for alignment with alignment features upon the backing plate, so that the sanding disk can on installation be aligned so that apertures within the sanding disk are matched with apertures within the backing plate.
Optionally the one or more alignment apertures may also serve as engagement means to mate with drive pins extending from the backing plate.
Optionally, one or more apertures are provided in the sanding disk in positions capable of matching air extraction apertures within a backing plate.
In a preferred aspect the perimeter of the sanding disk may be distorted from a circular shape by the provision of one or more gaps, most preferably in the form of segments, around from the circumference of the disk. Where a plurality of such gaps are provided it is preferred that they be symmetrically located to maintain balance in the disk. Preferably there are from three to eight gaps.
More preferably the number of gaps matches the number of non-concentric apertures adapted for viewing and ventilation and are located on radii between those on which the apertures are located.
Preferably each gap has the shape of a straight line joining one part of the circumference to another. Otherwise expressed, the gap is formed by removal of a segment of the disk.
Preferably the dimensions of the or each gap are adjusted so that when the sanding disk is rotated, it is possible to see through the disk in the zone outside that of the viewing/ventilation apertures, and as far as the edge.
Optionally this type of gap may be used advantageously in the procedure of cutting sanding disks from stock material, by bringing disk centres closer to each other and having common edges between adjacent disks, so as to minimize waste.
Optionally some or all gaps may have a curved outline.
A preferred curved outline is one that is drawn in towards the trailing edge of a viewing/ventilation aperture, thereby providing a narrowed or weakened zone capable of being torn should a projection engage with the viewing/ventilation aperture.
The surface of the abrasive disk can have a number of configurations. In a first embodiment the surface is provided by a coating of abrasive particles adhered to the surface of the disk by a binder material selected from cured resinous binders or metallic bonds. In a further embodiment the surface of the disk comprises a non-woven layer of fibers having bonded to the fibers a plurality of abrasive particles. Such non-woven layers are conventionally bonded to a backing material imparting a higher degree of dimensional stability to the whole disk structure.
In still another aspect the sanding disk may be provided with one or more peripheral foldsxe2x80x94or xe2x80x9cwing tipsxe2x80x9dxe2x80x94that are directed away from the abrasive surface, so that when the disk is rotated air is caused to move thereby further cooling the work area and directing the abraded material away.
In a related aspect a skirt may be provided around the guard of the angle grinder so as to confine the air brought into motion by the wing tips.
In yet another aspect the sanding disk is also provided with one or more shearing sites, xe2x80x9ctear zonesxe2x80x9d or deliberately provided points of weakness capable of disconnecting the disk from the drive means of the backing plate if the disk inadvertently engages with an object and attempts to transmit a high torque to the backing plate and to the angle grinder. A preferred shearing site comprises a weakened zone concentric with the mounting means or aperture.
Preferably this weakened zone is formed from a series of apertures cut into or through the material of the sanding disk. Optionally this weakened zone is formed from a series of slits cut into or through the material of the sanding disk.
Preferably a disk retaining nut tightened onto the arbor of the angle grinder is capable of retaining the torn-off sanding disk; preferably by means of a concentric, outwards-directed projection or the like provided towards the periphery of the disk retaining nut; the projection having a diameter large enough to include the weakened zone.
In any case the sanding disk should preferably remain substantially dynamically balanced about its axis of revolution.
The Backing Plate of the Invention
The abrasive disks described above are intended for use with a backing plate that is preferably made of a resilient material, and, also preferably, the material of the backing plate has a dark colour.
The backing plate includes at least one gap or aperture, positioned so as to be capable of alignment with the one or more non-concentric apertures adapted for viewing and ventilation provided within the sanding disk.
Preferably the or each gap or aperture in the backing plate is similarly provided with slanted or raked surfaces, and optionally each aperture may be provided with an air scoop.
Optionally the backing plate may be provided with further apertures substantially not capable of alignment with the non-concentric apertures adapted for viewing and ventilation in the sanding disk and one or more of the further apertures may be used for alignment purposes.
One or more of the further apertures may be used for purpose of driving the sanding disk, by means of engagement means held within said further apertures.
One or more of the further apertures may be used for air and material removal purposes; being connected to air extraction channels within the backing plate.
Preferably such extraction channels run outward from the removal aperture towards the periphery of the backing plate, so that in use air is moved through the channel by a centripetal force.
Yet further apertures in the backing plate may be provided in order to give the backing plate a weakened zone that may be ruptured if a protruding object is caught in a viewing/ventilation aperture.
Preferably the resilience of the combination of sanding disk and backing plate is sufficient to provide a significant flexibility of the actively abrading disk during use, so that more than just the edge of the disk can be in effective contact with a work surface.
In an alternative embodiment the backing plate itself is provided with clutch means capable of becoming disengaged from the drive shaft if the torque applied through the clutch means exceeds a preset limitxe2x80x94as for example if the backing plate inadvertently grips an object.
Another preferred embodiment of a clutch means is an overload clutch built into the material of the backing plate. This may comprise a shear pin.
Yet another preferred embodiment of a clutch means comprises a modification by lengthening of the shaft of a retaining nut and a modification by provision of a shaft for a thrust washer so that tightening the retaining nut against the thrust washer (when mounting a sanding disk and a backing plate forms an overload clutch acting in a manner analogous to a shear pin, allowing slippage. in the event of excess torque, between the backing plate and the retaining nut/backing washer assembly.
Preferably at least one hole in the backing plate and at least one hole in the sanding disk may be used in conjunction with a locating peg or pin to rotationally align the sanding disk on the backing plate so that the apertures are substantially in alignment. Preferably the locating peg or pin is removed after attachment of the sanding disk and before use.
Optionally a locating pin or projection included in a sanding disk and for alignment purposes inserted into the backing plate may also act during use as a shear pin.
Optionally an overload clutch may include serrations or the like capable of creating a vibration or noise against a projection when the clutch is slipping.
Preferably a sanding system comprising the backing plate of the invention also includes a guard for an angle grinder, adapted to protect the user from injury resulting from the spinning sanding disk and/or the backing plate; the guard comprising a protective cover mounted at least one of the threaded sockets for the gripping handle and projecting forwards between the sanding disk and the operator.
Preferably the guard is made of a tough clear plastics material; alternatively at least a part of it may be made of metal. Also preferably the guard is fixed in place. Alternatively however the guard may be adjustable and moved forwards or backwards from time to time, thereby acting as a gauge plate.